I wrote this Ace3 addon because I couldn't find anything that did this as simply. (I'm the Warlock class lead for my guild, and we seem to have an ungodly amount of Warlocks, so I really needed something to inform me when we were reaching the debuff limit.)
Basically, DebuffCounter creates a small frame displaying how many debuffs currently affect your Target. The number changes to red as the debuff limit is approached. You can also enable a second small counter for your Focus target.
Type /dc or /debuffcounter to bring up the options menu. It's also available under the Addons section of the Blizzard interface options menu.
Possible future features: FuBar support, Further customization of the frames display.
Feedback and suggestions appreciated.
v1.0 - Initial release
v1.1 - Bugfixes/code streamlining. Cleaner options menu. New features: can set to show only in party/raid, frame scaling, position reset, and transparency.
v1.3 - Configurable RaidWarning/Chat/Sound notices when Debuff limit is reached. Options streamlined. Bugs removed. Feature-complete!
v1.31 - Fixed silly colour-bug on Focus frame.